The present invention relates generally to an imaging system and method, and more particularly to a positron emission tomography (PET) detection system and method.
Conventional PET detectors use an array of scintillator crystals (i.e., scintillator pixels) optically coupled to a position-sensitive photo multiplier tube (xe2x80x9cPS-PMTxe2x80x9d) to achieve high spatial resolution and compactness. However, the edge areas 3 of the detection surface (i.e., the front face) of a PS-PMT 1 are radiation insensitive, as shown by gray bands in FIG. 1. This prevents the use of the entire PS-PMT front to detect radiation emitted by the scintillator pixels. Thus, only the radiation sensitive central area 5 of the front face of the PS-PMT 1 is used to detect radiation, such as photons, emitted by the scintillator pixels when the pixels are irradiated by the gamma rays. This is termed an xe2x80x9cedge problem.xe2x80x9d
The edge problem is particularly troublesome in PET systems having multiple detector rings, because a detection gap is formed at the ring edges. Since PS-PMTs are provided inside the rings, the ring edges contain the PS-PMT edge areas 3 which are radiation insensitive. When two rings are axially coupled, two PS-PMT radiation insensitive edge areas 3 are located adjacent to each other, exacerbating the edge problem. Thus, even if the detector rings contact each other, a detection gap (i.e., blank region) is formed in the image at the contact point of adjacent rings. In the prior art multiple detector ring PET system, in order to overcome the edge problem, an optical fiber (or optical waveguide) bundle 7 was used to couple radiation from the scintillator pixel array 9 to the PS-PMT 1, as shown in FIG. 2. However, the extra coupling, the absorption of the optical fiber, and most significantly the limited acceptance angle of optical fiber causes severe reduction of the number of photons (about 70-80% reduction) that can be detected by the PS-PMT. Alternatively, a single ring PET system may be used. However, such as system provides a limited axial coverage, which may be undesirable for some applications.
The PET system edge problem is illustrated in FIGS. 3 and 4 below. A Hamamatsu(copyright) R8520 PS-PMT with a resistor network chain for signal multiplexing and position dependent signal output for interaction positioning was used as a PS-PMT. Na-22 was used as the isotope source. The scintillator pixel array was attached to the PS-PMT with optical grease. The scintillator pixel array contained a 3xc3x973 array of mixed lutetium oxyorthosilicate (xe2x80x9cMLSxe2x80x9d) crystals having the following dimensions: 1.5xc3x971.5xc3x977 mm. Each crystal was wrapped with white PTFE tape leaving about a 0.2 mm gap between crystals.
When the 3xc3x973 array was attached at one quarter of the front face (i.e., detection surface) of the PS-PMT, all scintillator crystals were inside the radiation sensitive area 5 of the PS-PMT 1. In the position map (i.e., flood source image) shown in FIG. 3A, all nine crystals are clearly identified. The profile plots of the central row and column shown in FIGS. 3B and 3C, respectively, show very good separations between crystals. Thus, as expected, no edge problem was observed because all crystals were provided inside the radiation sensitive area of the PS-PMT.
However, when the same scintillator array (3xc3x973) was mounted at the edge of the face of PS-PMT 1, the left most column of the scintillator pixels was mounted adjacent to the radiation insensitive area 3 of the PS-PMT 1. Thus, the left most column of the scintillator pixels is not visible in the position map in FIG. 4A. Likewise, the pixel from the left most column was not visible in the profile plot of the lower row of pixels shown in FIG. 4B. Thus, the three pixels in this column either can""t be detected by PS-PMT, or can""t be distinguished from others because the photons from this column fell into the same anode of the PS-PMT as the photons from the middle column of pixels. This is an illustration of the edge problem.
In accordance with one preferred aspect of the present invention, there is a detector, comprising a first position sensitive radiation detector having a first radiation sensitive area and a second radiation insensitive area, a first scintillator having a first decay time, located adjacent to the first radiation sensitive area, and a second scintillator having a second decay time different than the first decay time, located adjacent to the second radiation insensitive area and being optically coupled to the first scintillator.
In accordance with another preferred aspect of the present invention there is provided a PET detection system, comprising a first detector ring, a second detector ring adjacent to and coaxial with the first detector ring, a first PS-PMT located on the first detector ring and a second PS-PMT located on the second detector ring, adjacent to the first PS-PMT, such that a first edge area of the first PS-PMT detection surface is adjacent to a first edge area of the second PS-PMT detection surface. The system further comprises a plurality of first scintillator pixels having a first decay time mounted adjacent to the detection surfaces of the first and the second PS-PMTs and at least one second scintillator pixel having a second decay time different from the first decay time, mounted adjacent to each of the first edge area of the first PS-PMT detection surface and the first edge area of the second PS-PMT detection surface. Each of the second scintillator pixels are optically coupled to an adjacent first scintillator pixel.
In accordance with another preferred aspect of the present invention, there is provided an imaging method, comprising receiving electromagnetic radiation at a first scintillator from a radiation source, emitting first radiation from the first scintillator having a first decay time and receiving the first radiation at a first radiation sensitive area of a first position sensitive radiation detector. The method also comprises receiving electromagnetic radiation at a second scintillator from the radiation source, emitting second radiation from the second scintillator having a second decay time different than the first decay time, and propagating the second radiation through the first scintillator to the first radiation sensitive area of the first position sensitive radiation detector. The method further comprises distinguishing a difference between the first decay time and the second decay time, and forming a position sensitive image from electromagnetic radiation received by the first and the second scintillators based on the difference between the first decay time and the second decay time.